1. Field of the Invention
The present invention relates to a method for making shadow masks to be incorporated in the cathode ray tube for color TV.
2. Description of the Prior Art
The shadow mask is manufactured by the following process. The normally produced cold-rolled steel sheet is subjected to re-cold roll to be less than 0.2 mm in thickness. Such re-rolled sheet is bored through in a photo-etching process and is then annealed at 650.degree.-950.degree. C. for a short period of time (hereinafter referred to as "final annealing"). The leveller process follower to decrease yield point elongation (Y.P.El) which causes stretcher strain, and to correct the shape of the steel sheet. Finally, the sheet is pressed into a shape corresponding to a front curve of the cathode ray tube.
In relation to the above mentioned shadow mask making process, the inventors have found that Al-killed ultralow C and cold rolled sheet including C and solute N in minimal amounts the is superior in photo-etching characteristics and press-formability, and is as a whole more suitable as the material for the shadow masks in comparison with the rimmed steel. The inventors have already proposed in Japanes patent application No. 53-133245 the method for making shadow maskes using a material of such Al killed ultra-low C cold rolled sheet.